1. Field of Endeavor
The present invention relates to carbon dioxide and more particularly to removal of carbon dioxide from the atmosphere and gas mixtures
2. State of Technology
Direct separation of CO2 from the atmosphere is an emerging technology option. Living creatures have already conquered this technologically difficult reaction by catalyzing the reaction of CO2 to CO3H— with carbonic anhydrase. In recent years a growing awareness of CO2 atmospheric levels sparked interest in developing rapid ways to absorb carbon dioxide from industrial gas streams. Most industrial separation processes for CO2 involve a liquid in which the dissolved CO2 ionizes, greatly increasing its solubility and absorption rate. The slow step in such processes is well known to be the formation of carbonic acid. This reaction controls the uptake of carbon dioxide by the ocean because it is just slow enough to cause a significant mass transfer limitation at the water's surface. This mass transfer limitation also applies to industrial gas separations and results in overall decreases in rate of factors in excess of 1000× over that which could be obtained if the hydration of the CO2 were not the rate limiting step. Speeding such processes through the use of catalysts or enzymes would permit smaller and less expensive separation processes to remove CO2 from industrial gas emissions, and be fast enough to permit removal of CO2 from the atmosphere.
In recent years a growing awareness of CO2 atmospheric revels sparked interest in. developing rapid ways to absorb carbon dioxide from industrial gas streams. Most industrial separation processes for CO2 involve a liquid in which the dissolved CO2 ionizes, greatly increasing its solubility and absorption rate. The slow step in such processes is well known to be the formation of carbonic acid. This reaction controls the uptake of carbon dioxide by the ocean because it is just slow enough to cause a significant mass transfer limitation at the water's surface. This mass transfer limitation also applies to industrial gas separations and results in overall decreases in rate of actors in excess of 1000× over that which could be obtained if the hydration of the CO2 were not the rate limiting step. Speeding such processes through the use of catalysis or enzymes would permit smaller and less expensive separation processes to remove CO2 from industrial gas emissions, and could even conceivably be fast enough to permit removal of CO2 from the atmosphere.
Carbonic anhydrase (CA) efficiently catalysis the reversible hydration of CO2 to carbonic acid. In erythrocytes, its rate kinetics surpasses the CO2 diffusion rate out of the cell. It is a ubiquitous enzyme expressed in prokaryote, and eukaryote organisms. The HMM library and genome assignment server lists 33 CA homologs in the human genome. CAII is the most efficient of the three forms of CA. Deficiency of CAII is associated with renal tubular acidosis and brain calcification, while it also plays a role in bone readsorption. Since its discovery, it sparked great interest due to its highly efficient kinetics and its Zn2+ metal center.
Current research into the use of carbonic anhydrase for industrial CO2 capture has received limited publication partially due to the difficulty of maintaining viable enzyme in industrial processes. Trachtenberg et al uses a membrane-countercurrent system originally designed for spacecraft use. Bhattacharya et al uses a spray system with carbonic anhydrase in the spray. Azari and Nemat-Gorgani examined means of using the reversible unfolding of the enzyme, caused by heat, to attach it to more sturdy substrates for industrial use. Yan et al. incorporate single carbonic anydrase molecules in a spherical nanogel and report that greatly improved temperature stability with only moderate loss of activity. Applicants are investigating whether small catalytic mimelics of CA may be more attractive as components of industrial gas separation processes, Creating such mimetics requires knowledge of the catalytic mechanism and possible degradation mechanisms of the catalytic enter.
Experimental and theoretical research contributed to the current understanding of CA's reaction mechanism. Crystallographic studies showed the Zrt2 ion in the CAII binding site is chelated by three hislidine side-chains and a water molecule to yield a tetrahedral coordination geometry. The reaction is thought to occur in three steps: 1) deprotonation of the water ligand to form an activated hydroxyl group, 2) a nucleophilic attack from the hydroxyl oxygen to the carbon atom in CO2 to form an intermediate species, and 3) the displacement of bicarbonate by water, which re-starts the cycle.